In the fabrication of bolts, cap screws, pins and similar fastening devices numerous manufacturing operations may result in decreased integrity of the material. One critical area is the junction of the bolt head and bolt shaft. It has been found that operations such as drilling can result in the loss of some of the properities imparted by heat treating. Some fabricators may heat socket head cap screws before drilling holes used for retaining wires to lower the hardness and make drilling easier. Such practices will often result in no visible alteration and a Rockwell hardness test on the shaft may also show the metal to be hardened. However the head may actually be softened and the critical fillet at the head-shaft junction may be weakened. This area of the bolt is especially subject to failure from hydrogen embrittlement and numerous other metaluriegicial problems.
Non-metallic fasteners also have some similiar problems resulting from the nature of the stresses produced during use. Defects such as honey combing, bubbles, poor material set or others will result in failure of the material in the critical area joining the head and the shaft in both metallic and plastic materials.
Studs and other threaded fasteners may be tested to insure the integrity of the material in the area of thread run out. This area is a common failure location and this device can be adapted to test either one end or both ends at the same time if so desired.
Testing is normally performed by visual inspection, measuring the exterior dimensions and an edge test under load. One standard for the wedge test is the Socket Screw Standard published by the Socket Screw Products Bureau as SS-103. In this test a force is applied to one edge of the screw head by placing a wedge washer member under the screw head and subsequently appling a load on the shaft of the bolt. The resulting stress produces a concentrated tension load on one side of the shaft as well as a bending movement. The test is destructive by nature and the load at which failure occurs and location of the failure determine compliance with the test standards.
Although testing by the methods of SS-103 are an improvement over testing under uniform tension, there are several drawbacks to this method. First the tests are destructive and they do not have the versitility required to test fasteners of varying materials such as brass and plastic.